Firstly find a plastic container large enough to work. Then roughly trace around your flash head:

Fine tune with a craft knife until your flash head fits the hole:

Next I sprayed the rear black to make it look better, got a Bacofoil foil dish, cut slots into it, applied a good strong adhesive and secured it into the hole before cutting the flash head cut out:

To make the inner dome I went to my local poundshop I bought 10 white plastic bowls. I cut the bowl down to make it shallower and glued that onto a lid from another container, resulting in this:

This then gets secured to the foil dish with cable ties and a circle of foil to be added to the dome

End result:

OLD PICTURES OF ME!! I'VE SHAVED SINCE THESE !!

This isn't a finished article yet as Peter (Mannesty) mentioned about using a contoured dome rather than a flat one but I never got around to doing it so may have to update this article in due course when I do !